The present invention relates to the generation of frequency signals in communication applications such as very-high frequency (VHF) radios and ultra-high frequency (UHF) radios. More particularly, the present invention relates to a phase locked loop (PLL) filter which utilizes a tuned filter.
Wireless devices, such as, radios, often require several sources of waveforms or signals having particular characteristics. For example, aircraft receive and transmit radios often require several signals at specific frequencies and amplitudes to perform receiving, transmitting, mixing, tuning, modulating, and demodulation operations. Heretofore, wireless devices have relied on several oscillators or voltage-controlled oscillators (VCOs) to generate the signals at several specific frequencies. Oscillators and voltage-controlled oscillators are nonrotating devices capable of producing an output signal having a frequency determined by the characteristics of the device. Oscillators tend to be bulky and expensive.
Digital dividers or prescalers provide a convenient and inexpensive apparatus for generating wide band signals from a single voltage-controlled oscillator. However, digital dividers tend to be noisier than voltage-controlled oscillators. The noise associated with the divider prevents it from being used in certain circumstances as a signal source. For example, signals from digital dividers are too noisy to be used as transmit signals. The noise from the digital divider can also create spurious off-carrier frequency signals.
Thus, there is a need to reduce the number of oscillators utilized in a communication system, such as, in receiving and transmitting portions of a ratio. Further still, there is a need for an apparatus which allows a digital divider to be utilized as a frequency signal source. Even further still, there is a need for an apparatus which allows a digital divider to provide transmit signals.
The present invention relates to a phase locked filter including a first signal splitter having a source input, a first output, and a second output; a band pass filter; a second signal splitter having a filter input, a third output, and a fourth output; and a phase control circuit. The band pass filter has a signal input, a signal output, and a control input. The signal input is coupled to the first output. The band pass filter has a frequency response in accordance with a control signal at the control input. The filter input is coupled to the signal output. The phase control circuit has a first phase input, a second phase input, and a phase output. The first phase input is coupled to the second output, and the second phase input is coupled to the third output. The phase output is coupled to the control input.
The present invention further relates to a phase locked filter for use in a frequency synthesizer which includes a divider having a divider output. The phase locked filter includes a filter and a phase comparator circuit. The filter has a filter input, a control input, and a filter output. The filter input is coupled to the divider output. The filter has a frequency response related to a chosen frequency indicated by a control signal provided at the control input. The filter has a phase response related to the chosen frequency. The phase comparator circuit has a first phase input, a second phase input, and a phase output. The first phase input is coupled to the divider output. The second phase input is coupled to the filter output. The phase output is coupled to the control input. The phase comparator provides the control signal at the phase output to reduce a difference in phase between signals at the first phase input and at the second phase input.
The present invention still further relates to a phase locked filter for use in a frequency synthesizer. The phase locked filter includes a filter means for filtering a signal at an input, and a phase comparator means for receiving the signal and a filtered signal. The filter means provides the filtered signal at an output in accordance with a band pass frequency response related to a center frequency indicated by a control signal provided at a control input. The phase comparator means venerates the control signal in accordance with a phase difference between the signal and the filtered signal. The filter means has a phase response of zero at the center frequency. The phase comparator provides the control signal at the phase output to reduce the phase difference.
In one aspect of the present invention, a phase locked loop employs a voltage-tuned filter to reduce noise associated with a frequency signal provided by a divider. The voltage-tuned filter is controlled by a phase comparator which adjusts the center frequency of the band pass filter in accordance with the phase difference before and after the filter. The filter is preferably a band pass filter which has a zero-degree phase response at the center frequency and a greater phase response as the frequency shifts away from the center frequency. Therefore, the phase comparator adjusts the center frequency of the band pass filter to match the frequency signal provided by the divider.
According to another aspect of the present invention, the band pass filter is comprised of an inductor and a varactor. The varactor has a control input coupled to the phase comparator. The phase locked loop can be utilized with a divider or a direct digital synthesizer (DDS) to generate a frequency signal suitable for transmission by a radio, such as, a VHF or UHF aircraft radio.